Some states, including Texas, Utah and Louisiana, were not included in the study because they did not have consistent data, or they changed their methods for collecting data during the study period, the researchers noted.
Nationally, about one in eight preschoolers is obese in the United States, the CDC said. Young children who are overweight or obese are five times more likely to be overweight or obese as adults, compared with normal-weight children, the CDC says.
The new report analyzed information from about 12 million preschoolers who take part in federally funded mother- and child-nutrition programs.
In 2011, preschool obesity rates ranged from a low of 9.2 percent in Hawaii, to 16.6 in New Jersey and Rhode Island, and 17.9 percent in Puerto Rico.
While the researchers could not determine the reason for the decline seen in some states, it may reflect efforts to increase awareness about the importance of healthy nutrition and physical activity, and reduced screen-time for young children, the CDC said. Increases in breastfeeding, which some studies suggest protects against childhood obesity, may have also played a role.